Parenting to an array end cap
Solution without drivers:
What about using a curve to define the length of your array ? You could then hook the last control point of your curve to your ObjRight. The solution isn't perfect as you would have a space between the OjRight and the Endcap but with a proper set up you could mask the problem (moving the ObjRight with fix increments snap to the grid for example).

Solution with drivers:
This solution (probably the best) would be to drive the ObjRight location (or an empty that you can use as a parent for ObjRight) by the array count.

Hope it makes sens. Let me know if you need clarification.